How to create a link between two forms that are prerequisites of each other?

Let's say I have a list of Drivers and a list of Buses. Now in my interface I am able to create an instance of a Driver and an instance of a Bus. Like below:

However as you can see, they can reference from each other as in a Bus can have a Driver and a Driver can have a Bus.

The dilemma is that they can be pre-requisites of each other, depending on which form you fill-out first. How do I solve this dilemma so that I can create an instance of a Driver and an instance of a Bus without them having the need to reference from each other? And when I need to reference/link them, how will that 'linking' interface look like?

Do I "Assign Bus" under Driver? Or do I "Assign Driver" under Bus? Or there's no right or wrong of the two ways?

Same issue for any many-to-many relationships types.. E.g. If you have a Movie CMS, do you "Add Actors" under a Movie or do you "Add Movies" under an Actor? Or a Library CMS, do you "Add Author" under a Book or do you "Add Book" under an Author?